1. Technical Field
The present disclosure relates to a power amplifier, and more particularly, to a quadrature modulator that performs quadrature modulation using a switched capacitor power amplifier.
2. Description of the Related Art
A power amplifier amplifies a weak input signal and outputs a resultant amplified signal to provide a signal with a high strength necessary in wireless communication. This amplification operation generally consumes large electric power. In particular, in a wireless communication apparatus (for example, a portable telephone or the like) driven by a battery, the power consumption has a significant influence on a maximum run time, and thus it is necessary to increase a power efficiency to reduce the power consumption.
One known method of increasing the power efficiency is to use a Class-D power amplifier. The Class-D power amplifier is an amplifier that operates using saturation of a transistor such that a current is passed through the transistor only during a switching period, which makes it possible to achieve low power consumption and high power efficiency. This type of amplifier is advantageous for use in modulation method in which a large change in amplitude does not occur.
However, in recent years, to achieve a better spectrum efficiency, orthogonal frequency-division multiplexing (hereinafter referred to as OFDM) has been widely used as a modulation method. For example, the OFDM is used in a wireless local area network (WLAN). In the OFDM, a plurality of carriers with different frequencies are modulated, and the modulated carriers are multiplexed such that the carriers are orthogonal to each other.
In the OFDM, at timings when phases of carriers overlap, a high power peak relative to average power occurs. The ratio of the peak power to the average power is referred to as a peak average power ratio (PAPR), which may be as large as 10 dB depending on a situation. In the OFDM, by nature of the principle of its operation, the PAPR is large. Therefore, to suppress an influence of intersymbol interference caused by distortion, a linear amplifier is used. In this case, if parameters are set such that the peak power is equal to the saturation power of the power amplifier, when the power amplifier operates with the average power that is smaller than the saturation power, the power amplifier operates at an operating point shifted from an operating point that provides a high efficiency, and thus a reduction in power efficiency occurs. In a case where there is a large difference between the average power and the saturation power, to ease the reduction in the power efficiency, it is common to cut peak power within a range in which resultant distortion is lower than an allowable level thereby reducing the PAPR. The difference between the maximum power and the average power in the state in which the peak power is cut is referred to as a backoff. With increasing backoff, the efficiency of the power amplifier decreases.
To handle the above-described situation, it is known to use a digital power amplifier such as that disclosed, for example, in Chao Lu, Hua Wang, C H Peng, Ankush Goel, SangWon Son, Paul Liang, Ali Hwang, and George Chein “A 24.7 dBm All-Digital RF Transmitter for Multimode Broadband Applications in 40 nm CMOS” ISSCC2013 SESSION 19/WIRELESS TRANSCEIVERS FOR SMART DEVICES/19.3. In the digital power amplifier, quadrature modulation is employed instead of polar modulation that is difficult to achieve for a signal with a large signal bandwidth, and a balun is used to combine currents.
In the digital power amplifier disclosed in Chao Lu, Hua Wang, C H Peng, Ankush Goel, SangWon Son, Paul Liang, Ali Hwang, and George Chein “A 24.7 dBm All-Digital RF Transmitter for Multimode Broadband Applications in 40 nm CMOS” ISSCC2013 SESSION 19/WIRELESS TRANSCEIVERS FOR SMART DEVICES/19.3, power amplifiers, each of which allows a fixed current to flow when operated, are cascode-connected, and the number of power amplifiers in operation is controlled so as to be proportional to a digital code thereby controlling the total current. The currents are combined using a balun and a resultant combined current is output to a load thereby providing output power. The digital code is a baseband signal up-sampled by an up-sampling digital front end. Thus, in the digital power amplifier disclosed in Chao Lu, Hua Wang, C H Peng, Ankush Goel, SangWon Son, Paul Liang, Ali Hwang, and George Chein “A 24.7 dBm All-Digital RF Transmitter for Multimode Broadband Applications in 40 nm CMOS” ISSCC2013 SESSION 19/WIRELESS TRANSCEIVERS FOR SMART DEVICES/19.3, currents always flow, which results in an increase in consumption current and thus a reduction in a power efficiency.
To ease the problem described above, it is known to use a switched capacitor power amplifier, as disclosed, for example, in Sang-Min Yoo, Jeffrey, S. Walling, Eum Chan Woo, Benjamin Jann and David J. Allstat, “A Switched-Capacitor RF Power Amplifier” IEEE J. Solid-state Circuits, vol. 46, no. 12, pp. 2977-2987, December 2011. In this switched capacitor power amplifier, the amplitude of the output voltage of an amplifier is controlled by controlling the number of Class-D power amplifiers being in operation according to a digital code. The Class-D power amplifier is an amplifier that allows a current to flow only during a switching period, which results in an increase in power efficiency. The switched capacitor power amplifier disclosed in Sang-Min Yoo, Jeffrey, S. Walling, Eum Chan Woo, Benjamin Jann and David J. Allstat, “A Switched-Capacitor RF Power Amplifier” IEEE J. Solid-state Circuits, vol. 46, no. 12, pp. 2977-2987, December 2011 is used to control the amplitude in the polar modulation.